Since their commercial introduction in the mid-1980s, applications for rare earth-iron-boron magnets have continued to grow and this material has become a major factor in the global rare earth permanent magnet market. Among commercially available permanent magnets, Nd2Fe14B type magnets offer the highest maximum energy product (BH)max ranging from 26 to 48. Experimental versions have reported a (BH)max in excess of 55 MGOe.
Nd2Fe14B rare earth magnets exhibit the highest room temperature magnetic properties, which is the basis for the wide use. As noted above, high performance Nd2Fe14B-based permanent magnets provide high maximum energy products (BH)max. In addition, they offer large saturation magnetization (4πMS) and high intrinsic coercivity (MHC). That the Nd—Fe—B-type permanent magnets continue to offer the most promise for high magnetic performance rare earth permanent magnets is evident from FIG. 1.
Unfortunately, the Nd2Fe14B rare earth permanent magnets are notoriously brittle and susceptible to oxidation. Chipping, cracking and fracture often occur during grinding, assembly and even during operation of conventional Nd2Fe14B magnets. The fact that since these magnets cannot be machined and/or drilled imposes serious limitations on the shapes and uses available. The reject rate in production attributed to brittleness/lack of toughness runs generally from 10 to 20% and, on occasion, reaches 30%. The poor fracture toughness of current rare earth permanent magnets is illustrated in FIG. 2.
All sintered rare earth permanent magnets, SmCo5, Sm2Co17, and Nd2Fe14B, are brittle due to the intrinsically brittle intermetallic compounds used for these magnets. Machinable permanent magnets include:                (a) Fe—Cr—Co-type, which unfortunately exhibit low magnetic-performance,        (b) Pt—Co-type which are too expensive, and        (c) Bonded permanent magnets that exhibit dramatically reduced performance, i.e. loss of up to 50% magnetic performance comparing to their sintered counterparts.        
Improvement in the fracture toughness of the class of rare earth permanent magnets of the REFeB-type, while maintaining their high: 4πMS, MHC, and (BH)max, would not only improve their manufacturing efficiency and machinability, but it would also expand the market for this class of permanent magnets, by offering opportunities for new applications, new shapes, new uses, lower costs, etc.
Relevant prior art in this area includes: U.S. Pat. Nos. 4,402,770; 4,597,938; 4,710,239; 4,770,723; 4,773,950; 4,859,410; 4,975,130 and 5,110,377. Additional references include U.S. Pat. Nos. 3,558,372 and 4,533,408. Relevant literature references include:    M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, and Y. Matsuura, “New material for permanent magnets on a base of Nd and Fe”, Journal of Applied Physics, volume 55, pp. 2083-2087, 1984;    Y. Kaneko and N. Ishigaki, “Recent developments of high-performance NEOMAX magnets”, JMEPEG, VOLUME 3, PP. 228-233, 1994;    M. Sagawa and H. Nagata, “Novel processing technology for permanent magnets” IEEE Trans. Magn. Volume 29, pp. 2747-2752, 1993;    S. Hirosawa and Y. Kaneko, “Rare earth magnets with high energy products” in Proceedings of the 15th international Workshop on Rare Earth Magnets and Their Applications, volume 1, pp. 43-53, 1998;    M. Takahashi, et al, “High performance Nd—Fe—B sintered magnets made by the wet process”, J. Appl. Phys., volume 83, pp. 6402-6404, 1998;    J. J. Croat, J. F. Hebst, R. W. Lee, and F. E. Pinkerton, “Pr—Fe and Nd—Fe based materials: A new class of high performance permanent magnets”, J. Appl. Phys. Vol. 55, pp. 2078-2082, 1984;    J. F. Herbst, “R2Fe14B materials: Intrinsic properties and technological aspects”, Rev. Mod. Phys. Vol. 63, pp. 819-898, 1991;    Croat et al, “Pr—Fe and Nd—Fe-based Materials: A New Class of High-Performance . . . ”, J. Appl. Phys., 55(6), Mar. 15, 1984;    Sagawa et al, “New Material for Permanent Magnets on a Base of Nd and Fe”, J. Appl. Phys. 55(6), Mar. 15, 1984;    Koon et al, Crystallization of FeB Alloys with . . . “J. Appl. Phys. 55(66), Mar. 15, 1984;    Stadelmaier, “The Neodymium-Iron Permanent Magnet Breakthrough”, MMPA Workshop, January 1984;    Japanese High Technology, “Request for New Magnetic Material . . . ”, vol. 4, No. 5, Aug. 1984;    “Neomax—Neodymium-Iron-Magnet”, Sumitomo Special Metals Co., Ltd. Lee, “Hot-Pressed Neodymium-Iron-Boron Magnets”, Appl. Phys. Lett. (46(8) Apr. 15, 1985;    R. K. Mishra, “Microstructure of Melt-Spun Neodymium-Iron-Boron Magnets”, International Conference on Magnetism, 1985;    Sagawa et al., “Permanent Magnet Materials Based on the Rare Earth-Iron-Boron Tetragonal Compounds”, The Research Institute for Iron, Steel and Other Metals, Tohoku University, Japan;    Givord et al., “Magnetic Properties and Crystal Structure of R2Fe14B”, Solid State Comm., vol. 50. No. 6, February 1984, pp. 497-499;    Herbst et al., “Relationships Between Crystal Structure and Magnetic Properties in R2Fe14B”, Phys. Dept., General Motors Res. Lab., pp. 1-10; and    Croat et al., “High-Energy Product Nd—Fe—B Permanent Magnets”, App. Phys. Lett., 44(1), Jan. 1, 1984, pp. 148-149.
Various rare earth permanent magnets can be formed by pressing and sintering the powder or by bonding with plastic binders. Sintered Nd2Fe14B parts produce the highest magnetic properties. Unfortunately, Nd2Fe14B magnets are sensitive to heat and normally cannot be used in environments that exceed 150° C.
Compared to the SmCo 1:5 and 2:17 magnets, Nd2Fe14B magnets have an excellent value in terms of price per unit of (BH)max. Small shapes and sizes with high magnetic fields are one of the attractive features of Nd2Fe14B magnets. Today's commercial Nd2Fe14B-based magnets include combinations of partial substitutions for Nd and Fe, leading to a wide range of available properties.
Several different techniques are used to produce Nd2Fe14B-based magnets. One method is similar to that used for ceramic ferrite and sintered Sm—Co magnets. The alloys with appropriate composition are induction melted to ingots, which are then crushed and milled to powders of a few microns. The powder is formed into a desired shape by pressing under alignment field. The pressed green compacts are then sintered to full density and heat treated to obtain suitable magnetic properties.
A second process involves rapid quenching of a molten Nd2Fe14B-based alloy, using a “melt spinning” technique to produce ribbons, which are then milled to powder. While the crushed ribbon yields relatively large platelet-shaped powder particles, rapid quenching provides them with an extremely fine microstructure having grain boundaries that deviate from the primary Nd2Fe14B composition. Rapidly quenched powder is inherently isotropic. However, it can be consolidated into a fully dense anisotropic magnet by the plastic deformation that occurs in hot pressing. The fine microstructure makes this powder very stable against oxidation, making it easy to blend and form into a wide range of isotropic bonded magnets.
Nd2Fe14B powder tends to readily absorb hydrogen, which degrades the material into a very brittle powder. This response to hydrogen renders the powder more amenable to milling and is the basis for the hydrogenation, disproportionation, desorption and recombination process generally referred to as HDDR. The HDDR process provides Nd2Fe14B powder with an ultrafine structure with grains about the size of a single domain. Such HDDR powder can be hot pressed into a fully dense anisotropic magnet, or it can be blended and molded into an anisotropic bonded magnet.